Implement review comments from @yorickpeterse

1. Change multiple updates to a single `update_all`

2. Use cascading deletes

3. Extract an average function for the database median.

4. Move database median to `lib/gitlab/database`

5. Use `delete_all` instead of `destroy_all`

6. Minor refactoring

17 files changed, 147 insertions, 225 deletions

diff --git a/app/models/ci/pipeline.rb b/app/models/ci/pipeline.rb
index cbb4f60cd32..d0997fdec19 100644
--- a/app/models/ci/pipeline.rb
+++ b/app/models/ci/pipeline.rb
@@ -57,15 +57,13 @@ module Ci
       end
 
       after_transition [:created, :pending] => :running do |pipeline|
-        pipeline.merge_requests.each do |merge_request|
-          merge_request.metrics.record_latest_build_start_time!(pipeline.started_at) if merge_request.metrics.present?
-        end
+        MergeRequest::Metrics.where(merge_request_id: pipeline.merge_requests.map(&:id)).
+          update_all(latest_build_started_at: pipeline.started_at, latest_build_finished_at: nil)
       end
 
       after_transition any => [:success] do |pipeline|
-        pipeline.merge_requests.each do |merge_request|
-          merge_request.metrics.record_latest_build_finish_time!(pipeline.finished_at) if merge_request.metrics.present?
-        end
+        MergeRequest::Metrics.where(merge_request_id: pipeline.merge_requests.map(&:id)).
+          update_all(latest_build_finished_at: pipeline.finished_at)
       end
 
       before_transition do |pipeline|
@@ -292,8 +290,11 @@ module Ci
     # Merge requests for which the current pipeline is running against
     # the merge request's latest commit.
     def merge_requests
-      project.merge_requests.where(source_branch: self.ref).
-        select { |merge_request| merge_request.pipeline.try(:id) == self.id }
+      @merge_requests ||=
+        begin
+          project.merge_requests.where(source_branch: self.ref).
+            select { |merge_request| merge_request.pipeline.try(:id) == self.id }
+        end
     end
 
     private
diff --git a/app/models/concerns/database_median.rb b/app/models/concerns/database_median.rb
deleted file mode 100644
index 19a495a05b8..00000000000
--- a/app/models/concerns/database_median.rb
+++ /dev/null
@@ -1,76 +0,0 @@
-# https://www.periscopedata.com/blog/medians-in-sql.html
-module DatabaseMedian
-  extend ActiveSupport::Concern
-
-  def median_datetime(arel_table, query_so_far, column_sym)
-    case ActiveRecord::Base.connection.adapter_name
-    when 'PostgreSQL'
-      pg_median_datetime(arel_table, query_so_far, column_sym)
-    when 'Mysql2'
-      mysql_median_datetime(arel_table, query_so_far, column_sym)
-    else
-      raise NotImplementedError, "We haven't implemented a database median strategy for your database type."
-    end
-  end
-
-  def mysql_median_datetime(arel_table, query_so_far, column_sym)
-    query = arel_table.
-            from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name)).
-            project(Arel::Nodes::NamedFunction.new("AVG", [arel_table[column_sym]]).as('median')).
-            where(Arel::Nodes::Between.new(Arel.sql("(select @row_id := @row_id + 1)"),
-                                           Arel::Nodes::And.new([Arel.sql('@ct/2.0'),
-                                                                 Arel.sql('@ct/2.0 + 1')]))).
-            # Disallow negative values
-            where(arel_table[column_sym].gteq(0))
-
-    [Arel.sql("CREATE TEMPORARY TABLE IF NOT EXISTS #{query_so_far.to_sql}"),
-     Arel.sql("set @ct := (select count(1) from #{arel_table.table_name});"),
-     Arel.sql("set @row_id := 0;"),
-     query,
-     Arel.sql("DROP TEMPORARY TABLE IF EXISTS #{arel_table.table_name};")]
-  end
-
-  def pg_median_datetime(arel_table, query_so_far, column_sym)
-    # Create a CTE with the column we're operating on, row number (after sorting by the column
-    # we're operating on), and count of the table we're operating on (duplicated across) all rows
-    # of the CTE. For example, if we're looking to find the median of the `projects.star_count`
-    # column, the CTE might look like this:
-    #
-    #  star_count | row_id | ct
-    # ------------+--------+----
-    #           5 |      1 |  3
-    #           9 |      2 |  3
-    #          15 |      3 |  3
-    cte_table = Arel::Table.new("ordered_records")
-    cte = Arel::Nodes::As.new(cte_table,
-                              arel_table.
-                                project(arel_table[column_sym].as(column_sym.to_s),
-                                        Arel::Nodes::Over.new(Arel::Nodes::NamedFunction.new("row_number", []),
-                                                              Arel::Nodes::Window.new.order(arel_table[column_sym])).as('row_id'),
-                                        arel_table.project("COUNT(1)").as('ct')).
-                                # Disallow negative values
-                                where(arel_table[column_sym].gteq(zero_interval)))
-
-    # From the CTE, select either the middle row or the middle two rows (this is accomplished
-    # by 'where cte.row_id between cte.ct / 2.0 AND cte.ct / 2.0 + 1'). Find the average of the
-    # selected rows, and this is the median value.
-    cte_table.project(Arel::Nodes::NamedFunction.new("AVG",
-                                                     [extract_epoch(cte_table[column_sym])],
-                                                     "median")).
-      where(Arel::Nodes::Between.new(cte_table[:row_id],
-                                     Arel::Nodes::And.new([(cte_table[:ct] / Arel.sql('2.0')),
-                                                           (cte_table[:ct] / Arel.sql('2.0') + 1)]))).
-      with(query_so_far, cte)
-  end
-
-  private
-
-  def extract_epoch(arel_attribute)
-    Arel.sql("EXTRACT(EPOCH FROM \"#{arel_attribute.relation.name}\".\"#{arel_attribute.name}\")")
-  end
-
-  # Need to cast '0' to an INTERVAL before we can check if the interval is positive
-  def zero_interval
-    Arel::Nodes::NamedFunction.new("CAST", [Arel.sql("'0' AS INTERVAL")])
-  end
-end
diff --git a/app/models/cycle_analytics.rb b/app/models/cycle_analytics.rb
index ef3f1bbcab5..d5f1c754dad 100644
--- a/app/models/cycle_analytics.rb
+++ b/app/models/cycle_analytics.rb
@@ -1,5 +1,5 @@
 class CycleAnalytics
-  include DatabaseMedian
+  include Gitlab::Database::Median
 
   def initialize(project, from:)
     @project = project
@@ -122,7 +122,9 @@ class CycleAnalytics
 
     case ActiveRecord::Base.connection.adapter_name
     when 'PostgreSQL'
-      result.first['median'].to_f
+      result = result.first.presence
+      median = result['median'] if result
+      median.to_f if median
     when 'Mysql2'
       result.to_a.flatten.first
     end
diff --git a/app/models/cycle_analytics/queries.rb b/app/models/cycle_analytics/queries.rb
deleted file mode 100644
index 7bdf1fb6290..00000000000
--- a/app/models/cycle_analytics/queries.rb
+++ /dev/null
@@ -1,80 +0,0 @@
-class CycleAnalytics
-  class Queries
-    def initialize(project)
-      @project = project
-    end
-
-    def issues(options = {})
-      @issues_data ||=
-        begin
-          issues_query(options).preload(:metrics).map { |issue| { issue: issue } }
-        end
-    end
-
-    def merge_requests_closing_issues(options = {})
-      @merge_requests_closing_issues_data ||=
-        begin
-          merge_requests_closing_issues = MergeRequestsClosingIssues.where(issue: issues_query(options)).preload(issue: [:metrics], merge_request: [:metrics])
-
-          merge_requests_closing_issues.map do |record|
-            { issue: record.issue, merge_request: record.merge_request }
-          end
-        end
-    end
-
-    def issue_first_associated_with_milestone_at
-      lambda do |data_point|
-        issue = data_point[:issue]
-        issue.metrics.first_associated_with_milestone_at if issue.metrics.present?
-      end
-    end
-
-    def issue_first_added_to_list_label_at
-      lambda do |data_point|
-        issue = data_point[:issue]
-        issue.metrics.first_added_to_board_at if issue.metrics.present?
-      end
-    end
-
-    def issue_first_mentioned_in_commit_at
-      lambda do |data_point|
-        issue = data_point[:issue]
-        issue.metrics.first_mentioned_in_commit_at if issue.metrics.present?
-      end
-    end
-
-    def merge_request_merged_at
-      lambda do |data_point|
-        merge_request = data_point[:merge_request]
-        merge_request.metrics.merged_at if merge_request.metrics.present?
-      end
-    end
-
-    def merge_request_build_started_at
-      lambda do |data_point|
-        merge_request = data_point[:merge_request]
-        merge_request.metrics.latest_build_started_at if merge_request.metrics.present?
-      end
-    end
-
-    def merge_request_build_finished_at
-      lambda do |data_point|
-        merge_request = data_point[:merge_request]
-        merge_request.metrics.latest_build_finished_at if merge_request.metrics.present?
-      end
-    end
-
-    def merge_request_deployed_to_production_at
-      lambda do |data_point|
-        merge_request = data_point[:merge_request]
-        merge_request.metrics.first_deployed_to_production_at if merge_request.metrics.present?
-      end
-    end
-
-    private
-
-    def issues_query(created_after:)
-      @project.issues.where("created_at >= ?", created_after)
-    end
-  end
-end
diff --git a/app/models/deployment.rb b/app/models/deployment.rb
index 1e338889714..183d538a867 100644
--- a/app/models/deployment.rb
+++ b/app/models/deployment.rb
@@ -42,4 +42,35 @@ class Deployment < ActiveRecord::Base
 
     project.repository.is_ancestor?(commit.id, sha)
   end
+
+
+  def update_merge_request_metrics
+    if environment.update_merge_request_metrics?
+      query = project.merge_requests.
+              joins(:metrics).
+              where(target_branch: self.ref, merge_request_metrics: { first_deployed_to_production_at: nil })
+
+      merge_requests =
+        if previous_deployment
+          query.where("merge_request_metrics.merged_at <= ? AND merge_request_metrics.merged_at >= ?",
+                      self.created_at,
+                      previous_deployment.created_at)
+        else
+          query.where("merge_request_metrics.merged_at <= ?", self.created_at)
+        end
+
+      # Need to use `map` instead of `select` because MySQL doesn't allow `SELECT`ing from the same table
+      # that we're updating.
+      MergeRequest::Metrics.where(merge_request_id: merge_requests.map(&:id), first_deployed_to_production_at: nil).
+        update_all(first_deployed_to_production_at: self.created_at)
+    end
+  end
+
+  def previous_deployment
+    @previous_deployment ||=
+      project.deployments.joins(:environment).
+      where(environments: { name: self.environment.name }, ref: self.ref).
+      where.not(id: self.id).
+      take
+  end
 end
diff --git a/app/models/environment.rb b/app/models/environment.rb
index 33c9abf382a..49e0a20640c 100644
--- a/app/models/environment.rb
+++ b/app/models/environment.rb
@@ -43,4 +43,8 @@ class Environment < ActiveRecord::Base
 
     last_deployment.includes_commit?(commit)
   end
+
+  def update_merge_request_metrics?
+    self.name == "production"
+  end
 end
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 2403748d017..06af94adf5e 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -23,9 +23,9 @@ class Issue < ActiveRecord::Base
 
   has_many :events, as: :target, dependent: :destroy
 
-  has_one :metrics, dependent: :destroy
+  has_one :metrics
 
-  has_many :merge_requests_closing_issues, class_name: MergeRequestsClosingIssues
+  has_many :merge_requests_closing_issues, class_name: 'MergeRequestsClosingIssues'
   has_many :closed_by_merge_requests, through: :merge_requests_closing_issues, source: :merge_request
 
   validates :project, presence: true
@@ -202,7 +202,7 @@ class Issue < ActiveRecord::Base
   # From all notes on this issue, we'll select the system notes about linked
   # merge requests. Of those, the MRs closing `self` are returned.
   def closed_by_merge_requests(current_user = nil)
-    return [] if !open?
+    return [] unless open?
 
     ext = all_references(current_user)
 
diff --git a/app/models/issue/metrics.rb b/app/models/issue/metrics.rb
index 3c1fcea9e6f..4436696cc1a 100644
--- a/app/models/issue/metrics.rb
+++ b/app/models/issue/metrics.rb
@@ -13,10 +13,6 @@ class Issue::Metrics < ActiveRecord::Base
     self.save if self.changed?
   end
 
-  def record_commit_mention!(commit_time)
-    self.update(first_mentioned_in_commit_at: commit_time) if self.first_mentioned_in_commit_at.blank?
-  end
-
   private
 
   def issue_assigned_to_list_label?
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index e7af37a6c0a..2daaaf42da6 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -13,11 +13,11 @@ class MergeRequest < ActiveRecord::Base
   has_many :merge_request_diffs, dependent: :destroy
   has_one :merge_request_diff,
     -> { order('merge_request_diffs.id DESC') }
-  has_one :metrics, dependent: :destroy
+  has_one :metrics
 
   has_many :events, as: :target, dependent: :destroy
 
-  has_many :merge_requests_closing_issues, class_name: MergeRequestsClosingIssues
+  has_many :merge_requests_closing_issues, class_name:  'MergeRequestsClosingIssues'
   has_many :issues_closed, through: :merge_requests_closing_issues, source: :issue
 
   serialize :merge_params, Hash
@@ -513,7 +513,7 @@ class MergeRequest < ActiveRecord::Base
   # running `ReferenceExtractor` on each of them separately.
   def cache_merge_request_closes_issues!(current_user = self.author)
     transaction do
-      self.merge_requests_closing_issues.destroy_all
+      self.merge_requests_closing_issues.delete_all
       closes_issues(current_user).each do |issue|
         MergeRequestsClosingIssues.create!(merge_request: self, issue: issue)
       end
diff --git a/app/models/merge_request/metrics.rb b/app/models/merge_request/metrics.rb
index 8c42c830fac..697a2e303fb 100644
--- a/app/models/merge_request/metrics.rb
+++ b/app/models/merge_request/metrics.rb
@@ -8,16 +8,4 @@ class MergeRequest::Metrics < ActiveRecord::Base
 
     self.save if self.changed?
   end
-
-  def record_production_deploy!(deploy_time)
-    self.update(first_deployed_to_production_at: deploy_time) if self.first_deployed_to_production_at.blank?
-  end
-
-  def record_latest_build_start_time!(start_time)
-    self.update(latest_build_started_at: start_time, latest_build_finished_at: nil)
-  end
-
-  def record_latest_build_finish_time!(finish_time)
-    self.update(latest_build_finished_at: finish_time)
-  end
 end
diff --git a/app/services/create_deployment_service.rb b/app/services/create_deployment_service.rb
index aad5cc8a15b..e958e91d612 100644
--- a/app/services/create_deployment_service.rb
+++ b/app/services/create_deployment_service.rb
@@ -13,39 +13,13 @@ class CreateDeploymentService < BaseService
       deployable: deployable
     )
 
-    update_merge_request_metrics(deployment, environment)
+    deployment.update_merge_request_metrics
 
     deployment
   end
 
   private
 
-  def update_merge_request_metrics(deployment, environment)
-    if environment.name == "production"
-      query = project.merge_requests.joins("LEFT OUTER JOIN merge_request_metrics ON merge_request_metrics.merge_request_id = merge_requests.id").
-              where(target_branch: params[:ref], "merge_request_metrics.first_deployed_to_production_at" => nil)
-
-      previous_deployment = previous_deployment_for_ref(deployment)
-      merge_requests_deployed_to_production_for_first_time = if previous_deployment
-                                                               query.where("merge_request_metrics.merged_at < ? AND merge_request_metrics.merged_at > ?", deployment.created_at, previous_deployment.created_at)
-                                                             else
-                                                               query.where("merge_request_metrics.merged_at < ?", deployment.created_at)
-                                                             end
-
-      merge_requests_deployed_to_production_for_first_time.each { |merge_request| merge_request.metrics.record_production_deploy!(deployment.created_at) }
-    end
-  end
-
-  def previous_deployment_for_ref(current_deployment)
-    @previous_deployment_for_ref ||=
-      project.deployments.joins(:environment).
-      where("environments.name": params[:environment], ref: params[:ref]).
-      where.not(id: current_deployment.id).
-      first
-  end
-
-  private
-
   def find_or_create_environment
     project.environments.find_or_create_by(name: expanded_name) do |environment|
       environment.external_url = expanded_url
diff --git a/app/services/git_push_service.rb b/app/services/git_push_service.rb
index cde993221ff..c499427605a 100644
--- a/app/services/git_push_service.rb
+++ b/app/services/git_push_service.rb
@@ -190,6 +190,8 @@ class GitPushService < BaseService
 
   def update_issue_metrics(commit, authors)
     mentioned_issues = commit.all_references(authors[commit]).issues
-    mentioned_issues.each { |issue| issue.metrics.record_commit_mention!(commit.committed_date) if issue.metrics.present? }
+
+    Issue::Metrics.where(issue_id: mentioned_issues.map(&:id), first_mentioned_in_commit_at: nil).
+      update_all(first_mentioned_in_commit_at: commit.committed_date)
   end
 end
diff --git a/db/fixtures/development/17_cycle_analytics.rb b/db/fixtures/development/17_cycle_analytics.rb
index f51d28d9ebf..e882a492757 100644
--- a/db/fixtures/development/17_cycle_analytics.rb
+++ b/db/fixtures/development/17_cycle_analytics.rb
@@ -4,7 +4,7 @@ require './spec/support/test_env'
 class Gitlab::Seeder::CycleAnalytics
   def initialize(project, perf: false)
     @project = project
-    @user = User.find(1)
+    @user = User.order(:id).last
     @issue_count = perf ? 1000 : 5
     stub_git_pre_receive!
   end
diff --git a/db/migrate/20160824124900_add_table_issue_metrics.rb b/db/migrate/20160824124900_add_table_issue_metrics.rb
index 04dff26043d..3d296c7e884 100644
--- a/db/migrate/20160824124900_add_table_issue_metrics.rb
+++ b/db/migrate/20160824124900_add_table_issue_metrics.rb
@@ -5,12 +5,12 @@ class AddTableIssueMetrics < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # When a migration requires downtime you **must** uncomment the following
   # constant and define a short and easy to understand explanation as to why the
   # migration requires downtime.
-  # DOWNTIME_REASON = ''
+  DOWNTIME_REASON = 'Adding foreign key'
 
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an
@@ -25,7 +25,7 @@ class AddTableIssueMetrics < ActiveRecord::Migration
 
   def change
     create_table :issue_metrics do |t|
-      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, null: false
+      t.references :issue, index: { name: "index_issue_metrics" }, foreign_key: true, dependent: :delete, null: false
 
       t.datetime 'first_mentioned_in_commit_at'
       t.datetime 'first_associated_with_milestone_at'
diff --git a/db/migrate/20160825052008_add_table_merge_request_metrics.rb b/db/migrate/20160825052008_add_table_merge_request_metrics.rb
index fb5494c5e8b..d743134d9a2 100644
--- a/db/migrate/20160825052008_add_table_merge_request_metrics.rb
+++ b/db/migrate/20160825052008_add_table_merge_request_metrics.rb
@@ -5,12 +5,12 @@ class AddTableMergeRequestMetrics < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # When a migration requires downtime you **must** uncomment the following
   # constant and define a short and easy to understand explanation as to why the
   # migration requires downtime.
-  # DOWNTIME_REASON = ''
+  DOWNTIME_REASON = 'Adding foreign key'
 
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an
@@ -25,7 +25,7 @@ class AddTableMergeRequestMetrics < ActiveRecord::Migration
 
   def change
     create_table :merge_request_metrics do |t|
-      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: true, null: false
+      t.references :merge_request, index: { name: "index_merge_request_metrics" }, foreign_key: true, dependent: :delete, null: false
 
       t.datetime 'latest_build_started_at'
       t.datetime 'latest_build_finished_at'
diff --git a/db/migrate/20160915042921_create_merge_requests_closing_issues.rb b/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
index 02c28cbe0dd..049340f7085 100644
--- a/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
+++ b/db/migrate/20160915042921_create_merge_requests_closing_issues.rb
@@ -25,8 +25,8 @@ class CreateMergeRequestsClosingIssues < ActiveRecord::Migration
 
   def change
     create_table :merge_requests_closing_issues do |t|
-      t.references :merge_request, foreign_key: true, index: true, null: false
-      t.references :issue, foreign_key: true, index: true, null: false
+      t.references :merge_request, foreign_key: true, index: true, dependent: :delete, null: false
+      t.references :issue, foreign_key: true, index: true, dependent: :delete, null: false
 
       t.timestamps null: false
     end
diff --git a/lib/gitlab/database/median.rb b/lib/gitlab/database/median.rb
new file mode 100644
index 00000000000..3ede0c6acd9
--- /dev/null
+++ b/lib/gitlab/database/median.rb
@@ -0,0 +1,80 @@
+# https://www.periscopedata.com/blog/medians-in-sql.html
+module Gitlab
+  module Database
+    module Median
+      def median_datetime(arel_table, query_so_far, column_sym)
+        case ActiveRecord::Base.connection.adapter_name
+        when 'PostgreSQL'
+          pg_median_datetime(arel_table, query_so_far, column_sym)
+        when 'Mysql2'
+          mysql_median_datetime(arel_table, query_so_far, column_sym)
+        else
+          raise NotImplementedError, "We haven't implemented a database median strategy for your database type."
+        end
+      end
+
+      def mysql_median_datetime(arel_table, query_so_far, column_sym)
+        query = arel_table.
+                from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name)).
+                project(average([arel_table[column_sym]], 'median')).
+                where(Arel::Nodes::Between.new(Arel.sql("(select @row_id := @row_id + 1)"),
+                                               Arel::Nodes::And.new([Arel.sql('@ct/2.0'),
+                                                                     Arel.sql('@ct/2.0 + 1')]))).
+                # Disallow negative values
+                where(arel_table[column_sym].gteq(0))
+
+        [Arel.sql("CREATE TEMPORARY TABLE IF NOT EXISTS #{query_so_far.to_sql}"),
+         Arel.sql("set @ct := (select count(1) from #{arel_table.table_name});"),
+         Arel.sql("set @row_id := 0;"),
+         query,
+         Arel.sql("DROP TEMPORARY TABLE IF EXISTS #{arel_table.table_name};")]
+      end
+
+      def pg_median_datetime(arel_table, query_so_far, column_sym)
+        # Create a CTE with the column we're operating on, row number (after sorting by the column
+        # we're operating on), and count of the table we're operating on (duplicated across) all rows
+        # of the CTE. For example, if we're looking to find the median of the `projects.star_count`
+        # column, the CTE might look like this:
+        #
+        #  star_count | row_id | ct
+        # ------------+--------+----
+        #           5 |      1 |  3
+        #           9 |      2 |  3
+        #          15 |      3 |  3
+        cte_table = Arel::Table.new("ordered_records")
+        cte = Arel::Nodes::As.new(cte_table,
+                                  arel_table.
+                                    project(arel_table[column_sym].as(column_sym.to_s),
+                                            Arel::Nodes::Over.new(Arel::Nodes::NamedFunction.new("row_number", []),
+                                                                  Arel::Nodes::Window.new.order(arel_table[column_sym])).as('row_id'),
+                                            arel_table.project("COUNT(1)").as('ct')).
+                                    # Disallow negative values
+                                    where(arel_table[column_sym].gteq(zero_interval)))
+
+        # From the CTE, select either the middle row or the middle two rows (this is accomplished
+        # by 'where cte.row_id between cte.ct / 2.0 AND cte.ct / 2.0 + 1'). Find the average of the
+        # selected rows, and this is the median value.
+        cte_table.project(average([extract_epoch(cte_table[column_sym])], "median")).
+          where(Arel::Nodes::Between.new(cte_table[:row_id],
+                                         Arel::Nodes::And.new([(cte_table[:ct] / Arel.sql('2.0')),
+                                                               (cte_table[:ct] / Arel.sql('2.0') + 1)]))).
+          with(query_so_far, cte)
+      end
+
+      private
+
+      def average(args, as)
+        Arel::Nodes::NamedFunction.new("AVG", args, as)
+      end
+
+      def extract_epoch(arel_attribute)
+        Arel.sql(%Q{EXTRACT(EPOCH FROM "#{arel_attribute.relation.name}"."#{arel_attribute.name}")})
+      end
+
+      # Need to cast '0' to an INTERVAL before we can check if the interval is positive
+      def zero_interval
+        Arel::Nodes::NamedFunction.new("CAST", [Arel.sql("'0' AS INTERVAL")])
+      end
+    end
+  end
+end